Devices that function as sources, amplifiers, and detectors of terahertz (THz) and millimeter wave radiation are of great interest due to potential applications for systems that utilize such devices. For example, a spectroscopic system operating at such ranges of radiation can detect complex chemical and biological compounds, live objects, and other features. Current applications of THz spectroscopy also include such important areas as deep space monitoring, radar applications, etc. However, to date, there is lack of fast solid state devices operating at these frequencies that can be used to build relatively cheap, fast and compact spectroscopic systems. As a result, most THz sources are bulky, fairly effective, and can only operate at cryogenic temperatures.
Gunn effect oscillators and amplifiers are well known to successfully operate at room temperature. Commercially available Gunn effect devices are based on bulk three-dimensional Gallium Arsenic (GaAs) active elements with or without a controlling electrode, placed into the resonator. Due to specific material related issues of GaAs, these devices have frequency limitations in the THz frequency range making them practically useless even at sub-terahertz frequencies without frequency multipliers, and extremely inefficient at frequencies above 100 gigahertz (GHz). Also, Gunn effect devices with large cross-sections have significant coherency problems and skin effect related issues.
Gallium Nitride (GaN)-based devices are much more promising for high frequency oscillation applications because of their higher (about at least two times) saturation velocities, higher breakdown fields, and better thermal properties than GaAs-based devices. Additionally, high densities of the two-dimensional electron populations available in GaN-based heterostructures make it possible to achieve plasma wave and negative differential mobility related instabilities in the two-dimensional electron plasma, which addresses the skin effect issues. Also, in two-dimensional structures, it is possible to use controlling electrodes (gates) that allow changing the carrier densities and electric field distributions independently, thus eliminating the effect of parasitic series resistances.